It's 1970, England have been knocked out of the World Cup; the Beatles are wearing beards and kaftans, shortly to go the way of Busted. And the British film industry has collapsed, largely because a small group of greedy Saudi Arabian men have decided to hike up the worldwide price of oil, so forcing the big American studios to withdraw their co-production wad. Overnight, a raft of distinguished actors, writers and directors are out of work: people desperate to stay in movies in order to pay the mortgage (no matter how bad these movies might be).

Only one thing can save the film industry. Porn. But not, y'know, porn that's really pornographic. No, British porn. Porn that Celia Imrie can star in. Or Captain Birds Eye. Or Olive from On The Buses. Porn with a flash of tit and arse, a twang of a G-string and The Sweeney's Dennis Waterman burning his cock on a hot-water bottle.

So was born the sex comedy. The most tragic episode in British cinema, but also, weirdly, the most memorable. Movies with strange and fantastically mundane titles like Confessions Of A Window Cleaner and Adventures Of A Taxi Driver, shot in glamorous locations like an out-of-season Butlins or a driving school in Cheam. Sex comedies lasted just 10 years, from 1970 to 1980, and disappeared as quickly as they'd first appeared. Yet the amazing thing is that these cheap, ridiculous movies saved the British film industry, at their height grossing more than the Bond or Carry On films, of which they were a raunchier, infinitely more surreal offshoot.

The pre-cursor of the sex comedy was a 1968 feature called What's Good For The Goose, starring the original dirty old man, Norman Wisdom, as a middle-aged letch trying to get his leg over a nubile hippy-chick in swinging London. It was coy and terrible and about as sexy as an in-growing toenail, but inspired by this film, British directors felt emboldened to start making their own sex comedies with titles like Not Tonight Darling!, Snow White And The Seven Perverts, and Secrets Of A Door-To-Door Salesman (part-directed by an out-of-work Jonathan Demme, who later made Seven and Silence Of The Lambs. Demme was rejected by the producers after one scene, clearly because he was never going to cut it in the film business).

Against all (low) expectations, these films began doing incredibly well at the box office. More ludicrous (but strangely engrossing) films followed with the likes of Leslie Ash as a saucy night nurse; Veronica Doran from Coronation Street as a proto-Bridget Jones, a sex-starved 1970s singleton who sexually assaults black men, in Escort Girls; Captain Birds Eye as a bearded sex machine in Wife Swappers; and Hywel Bennett as a disabled antique dealer with a rampant prosthetic penis.

The slightly unhinged feel of these films was increased by a big steaming dollop of puerile humour, which came from the fact that many of the writers and directors (such as Nat Miller and Pete Walker) came from a music-hall background, big on fart gags and custard pies. Walker, director of the legendary Greta In Four Dimensions (the first ever porn film in 3D), even had a short career as a failed stand-up comedian. Aside from Greta, Walker also boasted he'd written the quickest script in film history, I Like Birds, in under six hours - with dialogue and everything. It cost less than the price of a Capri to shoot.

The films often played in double bills with an ancient horror film starring Boris Karloff, which was never half as scary as the sex comedy, or an art film (with a bit of nudity in it) like Fellini's Casanova, or The Bitter Tears Of Petra Von Kant (upper-market lezzie-action).

The don of sex comedies was Robin Askwith in the Confessions films. The plot was often written in the back of the transit van on the way to the shoot, and the same every... single... time. Cheeky cockney virgin (Robin) gets a job (say a window cleaner, or a taxi driver) allowing him to ogle and/or fondle as many birds as possible. A chance encounter with an older ie experienced woman (usually on a washing machine or in a Mini) educates Robin in the ways of un-doing bras and he journeys forth to pleasure the female world, with hilarious comic consequences for absolutely no one.

I've had an affection for bad sex comedies ever since a friend of mine at school called Danny found a video of David Sullivan's Come Play With Me hidden among his dad's collection of angling magazines, organising a private viewing while his mum was at Safeway. The curtains were drawn. The matt-black VCR turned on. At some point (possibly when Mary Millington starts massaging some old codger in the sauna in suspenders) Danny suggested we all hide behind a piece of MFI furniture and spank our respective monkeys.

Instead, we just watched in horrified fascination at the unfolding weirdness. Old men in goofy teeth and stained pyjamas ran across fields after naked girls, Benny Hill-style; sheds exploded with Y-fronts cascading down from the heavens; women sprayed whipped cream on their breasts while some ageing queen in a kimono watched on, applauding.

Our appetite was whetted. We saw Eskimo Nell, starring Christopher Biggins (yes, him), perhaps the most preposterous sex film of all time. A blue movie that satirises the idea of making a preposterous blue movie, with scenes including a gay western and a kung fu musical. Even aged 12, we knew this wasn't really how porn should be. Possibly it was also, well, a little gay, whatever that may be. Many sex comedies were made by gay writers and directors, who couldn't stand making heterosexual blue movies, so set out to take the piss by deliberately camping up the plot and dialogue. Films like Confessions Of A Window Cleaner spend as much time with the camera lingering on Robin Askwith's arse up a ladder, as they do on female cleavage.

The furtive, secretive way in which these films approach sex paralleled the embarrassment the audience felt about going to see the films. Sex comedies are steeped in sexual frustration, the subject of Michael Powell's classic 1959 study of voyeurism, Peeping Tom, under whose slightly scary shadow sex comedies lurked.

This darker side of British porn - of an underbelly of respectable men like lawyers and bank managers getting off on sex and violence - is given a mad, grinning spin in sex comedies, but it's still there. Though films like the Confessions series were shown in high-street cinemas, more out-there titles like David McGillivray's cult S&M film House Of Whipcord (with whipped naked women being chased across Broadmoor by mental patients) were shown in "specialist" cinemas.

It was here that the so-called dirty mac brigade would congregate in the dark, the aisle rocking in unison. Some of these cinemas were so dirty, rats ran down the aisles carrying discarded crisp packets. Seasoned directors and producers of these films, like Stanley Long and Arnold Miller, didn't want to make tawdry wank movies. They really wanted to make art films. In between projects, Long shot part of Roman Polanski's Repulsion and Miller produced The Witchfinder General. Martin Campbell, who made Sex Thief with blue-movie veteran Christopher Biggins, went on to make Bond movies. Norman Cohen, who made the Confessions films, spent his money on a string of unsuccessful avant garde films.

It was 10 years before the British audience woke up and realised that sex comedies were absolute shit. But in the interim, these films did what they set out to do: keep thousands of people in work. Ironically, it was proper porn and VCRs that finally killed off the British sex comedy. US porn bonkbusters like Deep Throat and Debbie Does Dallas made the sight of Lynda Bellingham from the Oxo ads with her tits out in the back of a Mini seem a little lame. Now, however, in a world saturated in high-street sex, these strange little films look oddly fresh. Not just a bizarre historical capsule from the 1970s but a missive from a land where casual sexism, racism, pathetic humour and fumbled sex were turned into an unwitting art-form. By the way, you can get 'em all on DVD in a leather-bound box set. I know I will.